Task 是什么 ？
Task 是一个类， 它表示一个操作不返回一个值，通常以异步方式执行。
System.Threading.Tasks.Task<TResult>
大多数情况下，lambda 表达式用于指定的任务是执行的工作。
Task 怎么用 ？
Run(Action) 方法来启动循环，然后显示循环迭代数的任务︰
using System; using System.Threading.Tasks; public class Example { public static void Main() { Task t = Task.Run( () => { // Just loop. int ctr = 0; for (ctr = 0; ctr <= 1000000; ctr++) {} Console.WriteLine("Finished {0} loop iterations", ctr); } ); t.Wait(); } }
using System; using System.Threading.Tasks; public class Example { public static void Main() { Task t = Task.Factory.StartNew( () => { // Just loop. int ctr = 0; for (ctr = 0; ctr <= 1000000; ctr++) {} Console.WriteLine("Finished {0} loop iterations", ctr); } ); t.Wait(); } }
您可以同步调用线程的执行，以及异步任务它启动通过调用 Wait 方法来等待要完成的一个或多个任务。
Wait 方法将一直阻塞调用线程直到单一类实例都已完成执行。
Thread.Sleep 方法进入睡眠状态两秒钟。
using System; using System.Threading; using System.Threading.Tasks; class Program { static Random rand = new Random(); static void Main() { // Wait on a single task with no timeout specified. Task taskA = Task.Run( () => Thread.Sleep(2000)); Console.WriteLine("taskA Status: {0}", taskA.Status); try { taskA.Wait(); Console.WriteLine("taskA Status: {0}", taskA.Status); } catch (AggregateException) { Console.WriteLine("Exception in taskA."); } } }
由于下面的示例将启动一个任务，它在睡眠两秒钟，但定义的一秒的超时值，调用线程受到阻止，直到超时到期和之前的任务已完成执行。
using System; using System.Threading; using System.Threading.Tasks; public class Example { public static void Main() { // Wait on a single task with a timeout specified. Task taskA = Task.Run( () => Thread.Sleep(2000)); try { taskA.Wait(1000); // Wait for 1 second. bool completed = taskA.IsCompleted; Console.WriteLine("Task A completed: {0}, Status: {1}", completed, taskA.Status); if (! completed) Console.WriteLine("Timed out before task A completed."); } catch (AggregateException) { Console.WriteLine("Exception in taskA."); } } }
Wait 方法终止。
此示例随后显示所有三个任务的状态的信息。
using System;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
var tasks = new Task[3];
var rnd = new Random();
for (int ctr = 0; ctr <= 2; ctr++)
tasks[ctr] = Task.Run( () => Thread.Sleep(rnd.Next(500, 3000)));
try {s
int index = Task.WaitAny(tasks);
Console.WriteLine("Task #{0} completed first.\n", tasks[index].Id);
Console.WriteLine("Status of all tasks:");
foreach (var t in tasks)
Console.WriteLine(" Task #{0}: {1}", t.Id, t.Status);
}
catch (AggregateException) {
Console.WriteLine("An exception occurred.");
}
}
}
下面的示例创建十个任务，等待所有十若要完成，然后显示其状态。
using System;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
// Wait for all tasks to complete.
Task[] tasks = new Task[10];
for (int i = 0; i < 10; i++)
{
tasks[i] = Task.Run(() => Thread.Sleep(2000));
}
try {
Task.WaitAll(tasks);
}
catch (AggregateException ae) {
Console.WriteLine("One or more exceptions occurred: ");
foreach (var ex in ae.Flatten().InnerExceptions)
Console.WriteLine(" {0}", ex.Message);
}
Console.WriteLine("Status of completed tasks:");
foreach (var t in tasks)
Console.WriteLine(" Task #{0}: {1}", t.Id, t.Status);
}
}
WaitAll 方法调用，并且是处理 try/catch 块。
using System;
using System.Threading;
using System.Threading.Tasks;
public class Example
{
public static void Main()
{
// Create a cancellation token and cancel it.
var source1 = new CancellationTokenSource();
var token1 = source1.Token;
source1.Cancel();
// Create a cancellation token for later cancellation.
var source2 = new CancellationTokenSource();
var token2 = source2.Token;
// Create a series of tasks that will complete, be cancelled,
// timeout, or throw an exception.
Task[] tasks = new Task[12];
for (int i = 0; i < 12; i++)
{
switch (i % 4)
{
// Task should run to completion.
case 0:
tasks[i] = Task.Run(() => Thread.Sleep(2000));
break;
// Task should be set to canceled state.
case 1:
tasks[i] = Task.Run( () => Thread.Sleep(2000),
token1);
break;
case 2:
// Task should throw an exception.
tasks[i] = Task.Run( () => { throw new NotSupportedException(); } );
break;
case 3:
// Task should examine cancellation token.
tasks[i] = Task.Run( () => { Thread.Sleep(2000);
if (token2.IsCancellationRequested)
token2.ThrowIfCancellationRequested();
Thread.Sleep(500); }, token2);
break;
}
}
Thread.Sleep(250);
source2.Cancel();
try {
Task.WaitAll(tasks);
}
catch (AggregateException ae) {
Console.WriteLine("One or more exceptions occurred:");
foreach (var ex in ae.InnerExceptions)
Console.WriteLine(" {0}: {1}", ex.GetType().Name, ex.Message);
}
Console.WriteLine("\nStatus of tasks:");
foreach (var t in tasks) {
Console.WriteLine(" Task #{0}: {1}", t.Id, t.Status);
if (t.Exception != null) {
foreach (var ex in t.Exception.InnerExceptions)
Console.WriteLine(" {0}: {1}", ex.GetType().Name,
ex.Message);
}
}
}
}
四: 并行迭代 Task.Parallel
API 会判定同时执行多少个线程效率最高。效率由一个爬山算法来决定。
一个parallel并行的例子